For general information on martial arts, check out www.martialway.com
Don't waste your time with TKD. Outside of Korea it's 97% crap and even there it's still iffy.
BJJ will definitely get you into shape. You can go at pretty much your own pace, and as you get more gas you can go for longer. Most of the classes involve sparring or "rolling" so it's a great workout.
Don't worry about getting your ass kicked. If it's a good school the students will respect each other and you won't get injured. Just remember to tap when someone gets you in a submission.
You're lucky to have a Mixed Martial Arts school in your town. Some people have to settle for something else when that's what they really want.
Go to the mixed martial arts school, it's definitely the best in town.